If you log in and get stuck in a "loop" where you constantly are asked to log in or register, try checking one of two things:
This site uses cookies only to check if you are both registered and successfully logged in. No information about you or your visit is collected other than what is asked when you register.
For example, Explorer's cookie feature can usually be found under "Tools > Internet Options". Check your browser's "Help" settings for more details.
After changing your settings, you may have to quit the browser and all open windows.
Most major browsers feature an option to browse privately. Since this option typically disables cookies it will also prevent you from successfully logging into websites that require registration or log in. Check to make sure this feature is turned off.
If you are clicking links in an E-Mail message -- some web-based E-Mail clients (like Hotmail, MSN, etc.) that open links in a frame can interfere and make it difficult for login-based sites to function properly. If you fall into this scenario try opening a new browser window and copy/paste the link from the E-Mail into that new browser window.